White Americans have the same crime rate as white Europeans. Black Americans have the same crime rate as black Africans. I think your hypothesis doesn't hold water.
Not at all. I have never argued that blacks are monolithic or that race is essential. Igbos are well known to be high IQ ("The Jews of Africa"). African Americans were not imported as slaves from Nigera, but from what is now Ghana, Liberia, Sierra Leone, etc. Different pop.